Biochar was prepared from parthenium hysterophorus biomass following the procedures described in (Sutar and Jadhav, 2024). Precisely, the precursor material was collected from Addis Ababa, Ethiopia. The collected parthenium hysterophorus sample was thoroughly washed and rinsed with tap and distilled water respectively. Then, the sample was allowed to dry in sunlight for 5 days and then placed in the oven at 105 ℃ overnight. Thereafter, the moisture freed sample was size reduced and pyrolyzed at a temperature of 500 ℃ for 2 h following the procedures described in Fito et al. as shown in Figure 1, where A is a raw parthenium hysterophorus stem, B is dried powdered sample and C is parthenium hysterophorus biochar (Fito et al., 2023b).

Additionally, magnetite was synthesized using the co-precipitation method using NaNO₃ and FeSO₄.7H₂0. This method offers a simple and facile approach for the synthesis of Fe₃O₄ particles with superparamagnetic characteristics and aggregated morphology in high purity. Likewise, coprecipitation method was used for the preparation of magnetite biochar composite derived from parthenium hysterophorus utilizing NaOH as a precipitating agent. The synthesis process involves loading iron oxide particles onto the surface of the biochar through a simple coprecipitation method following the procedures described elsewhere (Frolova, 2019). Figure 2A, B are magnetite and magnate biochar composite derived from parthenium hysterophorus respectively.

The mass addition method was used to determine the pH point of zero charge (pHpzc) for composite adsorbent. This method involves adding salt to composite adsorbent containing samples and measuring their pH after 48 h following a procedure described elsewhere in (Abewaa et al., 2023a; Fito et al., 2023a). Scanning Electron microscope (SEM), Fourier Transform Infrared Spectroscopy (FTIR), and X-ray Diffraction (XRD) characterization procedures are commonly used for the analysis of adsorbent materials. SEM was used to study the morphology and surface structure of adsorbent material (Muttil et al., 2023; Sahu et al., 2023; Veeresh et al., 2023). FTIR was used to identify the functional groups present in the composite adsorbent. XRD analysis was used to determine the crystalline and amorphous nature of the composite adsorbent prepared from magnetite and parthenium hysterophorus biochar. These characterization techniques provide valuable information about the structure, surface properties, and composition of the magnetic biochar, which is important for understanding its adsorption properties and potential applications.

The experimental design employed for the adsorption of MB onto magnetic biochar consisted of four independent variables, each with two levels. The optimization of MB adsorption was achieved through the utilization of a full factorial design. The operating parameters encompassed pH values of 3 and 9, contact times of 20 and 40 min, initial MB concentrations of 100 and 150 mg/L, and adsorbent dosages of 10 and 40 mg per 100 mL, as outlined in Table 1. The batch adsorption process for MB from an aqueous solution was conducted in accordance with previously described procedures. The concentration of methylene blue within the water was determined via UV-Vis spectrophotometry, specifically at a maximum wavelength of 6,658 nm. A comprehensive calibration curve was established to facilitate this analysis, which is applicable across a broad range of methylene blue concentrations. Finally, the removal efficiency (RE, %) and adsorption capacity (QE, mg/g) are expressed by Eqs 1 and 2 respectively. Here, Co (mg/L) and Cf (mg/L) denote the initial and final concentrations of the dye, m represents the mass of the adsorbent in grams, and V signifies the volume of the solution (Obayomi et al., 2023; Wang et al., 2023; Guan et al., 2024). R E =   C o − C f C o × 100 % (1) Q e = C O − C f × V m (2)

An adsorption isotherm describes the equilibrium of adsorption of a substance on a solid surface at a constant temperature. It represents the relationship between the amount of adsorbate adsorbed on an adsorbent and the concentration of adsorbate in solution when equilibrium is reached (Mozaffari Majd et al., 2022). Moreover, adsorption isotherms are mathematical equations that quantify the affinity of the adsorbate for the adsorbent (Saleh, 2022). They can accurately describe various adsorption scenarios, including those that consider lateral inter-adsorbate interactions, molecular size, surface heterogeneity, and mobile adsorption (Mozaffari Majd et al., 2022). Additionally, they are used to model experimental equilibrium data and optimize the adsorbent mass in the adsorption process (Dehghani et al., 2021). Adsorption isotherms are essential in understanding the adsorption mechanism and catalytic processes (Russo et al., 2021). Different adsorption models, such as the Freundlich, Langmuir and Temkin isotherms, can be used to fit the experimental data and determine the adsorption capacity and intensity of the adsorbent. In this study, the study of adsorption isotherm was carried out at constant pH (9), contact time (40 min) and adsorbent dosage (0.04 g/100 mL) while varying initial dye concentration from 100–150 (100, 110, 120,130, 140 and 150 mg/L). Freundlich, Langmuir and Temkin isotherm models used in this research are shown in Table 2.

Adsorption kinetics is the study of the rate at which adsorption occurs. Various mathematical models have been developed to describe this process, including exponential models (Revellame et al., 2020). The kinetics of adsorption can be influenced by factors such as the type of adsorbent and the nature of the adsorbate (Benjelloun et al., 2021). The pseudo first order, pseudo second order and intraparticle diffusion are commonly used to analyze adsorption kinetics, and they can be applied to both local and non-local kinetic equations (Xu et al., 2022). The adsorption process can be divided into two stages: a rapid stage leading to a first phase pseudo-equilibrium, followed by a slower stage leading to a second phase pseudo-equilibrium (Amah and Agunwamba, 2022). Understanding adsorption kinetics is important for various applications, including environmental remediation and the design of adsorption units. In this study, the kinetics investigation of adsorption of MB onto magnetic biochar was carried at a constant adsorbent dosage of 0.04 g/100mL, initial dye concentration of 100 mg/L and pH value of 9 while varying the contact time from 10–40 min (10,15, 20, 25, 30,35 and 40 min). Pseudo first order, pseudo second order and intraparticle diffusion models are presented in Table 3.

Adsorption thermodynamics involves the study of the thermodynamic properties of adsorption processes, such as equilibrium constants, Gibbs free energy change ( ∆ G, kJ/mol) enthalpy change ( ∆ H, kJ/mol), and entropy change ( ∆ S, kJ/mol. K). These properties are important for understanding and predicting the mechanism of the adsorption process. Adsorption is a widely used technique in various industries, including water treatment, petroleum refining, gas separation, and air purification. Overall, the study of adsorption thermodynamics in conjunction with adsorption kinetics is essential for a comprehensive understanding of adsorption process. In this research, the investigation of adsorption thermodynamics was done at varying temperatures of 25–45 ℃ . However, constant values of pH (9), contact time (40 min), initial MB concentration (100 mg/L) and adsorbent dosage of 0.04 g/100 mL were used during thermodynamic investigation. The essential thermodynamic parameters are determined from Eqs 3–6 (Hummadi et al., 2022; Panda et al., 2021; salah omer et al., 2022). ∆ G = − RTln k c (3) K C = q e C e (4) ln ⁡ k c = ∆ S R − ∆ H RT (5) ∆ G = ∆ H − T ∆ S (6) Where K_C is the thermodynamic constant, R is the universal gas constant (8.314 J/mol. K), and T is the absolute temperature in K. From the plot between ln k_c and 1/T, the value of ΔH and ΔS was calculated from the slope and intercept obtained.

Composite adsorbents are effective in removing organic dyes and heavy metal ions from water due to their large surface areas (Futalan et al., 2022). However, pollutants accumulate, leading to a decline in adsorption performance and hazardous solid waste (Jagadeesh and Sundaram, 2023). Recycling waste adsorbents through chemical methods is economically and environmentally effective (Baskar et al., 2022). This study investigates the regenerability and reusability potential of composite adsorbents using a chemical regeneration method using NaOH, which is widely used due to its simplicity and effectiveness. Precisely, pollutant loaded adsorbent was added in to the solution containing 0.1 M NaOH and shaken at 250 rpm for 2 h in order to desorb the pollutant attached to the surface of the composite material. Thereafter, the pollutant desorbed adsorbent material was dried in the oven at 105 ℃ for 24 h in order to remove the moisture. Finally, MB desorbed magnetic biochar was repeatedly used for five cycles for the adsorption of MB from an aqueous solution following the procedures described in batch adsorption experiments.

The pHpzc of the magnetic biochar derived from parthenium hysterophorus was depicted in Figure 3 and determined to be 6.7. The pHpzc values aid in understanding adsorbent material surface charge, stability, electrolyte interaction, suspension rheology, and ion exchange capacity, and characterize magnetic biochars’ adsorption properties at zero charge. Accordingly, the surface of the adsorbent material is positively charged below pH of 6.7 and negative charges dominate the surface of the adsorbent materials above pH of 6.7. This indicated that cationic dyes like MB are sufficiently adsorbed above pH of 6.7 and the reduction in adsorption capacity is expected below pH of 6.7 due to the repulsion between cationic MB dyes and positively charged magnetic surface. A similar finding is reported for magnetite impregnated biochar of parthenium hysterophorus (Fito et al., 2023b).

The functional groups of the composite adsorbent composed of magnetite and parthenium hysterophorus biochar were assessed using FTIR analysis. The number of the functional groups of the adsorbent is presented in Figure 4. This adsorbent showed the presence of many functional groups which indicates the promising adsorbent to interact with many pollutants in the wastewater. The band at 3,275 cm⁻¹ is attributed to O–H stretch vibration, 2,920 cm⁻¹ corresponds to C-H stretch vibration for alkanes, 2,114 cm^–1, 1,604 cm⁻¹ corresponds to C=O stretch vibration of carboxylate, 1,317 cm⁻¹ is attributed to C-H bending vibrations (C-H deformation), 1,010 cm⁻¹ corresponds to C-O stretches. The peak at 769 cm^-1, is attributed to C-H bending vibration. The peak at 520 cm^-1 corresponds to Fe-O indicating successful impregnation of magnetite onto parthenium hysterophorus-derived biochar (Moges et al., 2022; Abewaa et al., 2023b; Fito et al., 2023b; Fito et al., 2023c; Fito and Ebrahim, 2023).

The X-ray diffraction technique was employed to investigate the crystalline structure of the composite consisting of magnetite and biochar. The findings of the XRD analysis are illustrated in Figure 5. The materials exhibited crystalline properties at specific angles (2θ) of 30.16, 35.53, 43.14, 57.11 and 63.12 corresponding to the peaks (220), (311), (400), (422), and (511), respectively. The predominant peak pattern observed in the composite materials can be attributed primarily to the presence of magnetite in the adsorbent. Given its amorphous nature, the XRD pattern is not expected to reveal the emergence of peaks related to parthenium hysterophorus-derived biochar. These outcomes align closely with the findings reported by Fito et al. (2023), where magnetic biochar material exhibited peaks at angles of 30.15, 35.51, 43.16, 53.32, and 57.08 (Fito et al., 2023b).

The examination of the surface morphology of the biochar-magnetite composite obtained from a sample of parthenium hysterophorus was conducted using Scanning Electron microscope (SEM). The SEM analysis produced an image, as presented in Figure 6. Within this study, the surface of the composite material was magnified by a factor of 300, enabling the observation of distinct pore sizes and their distributions. It was observed that the composite adsorbent possessed highly irregular surfaces characterized by a significant level of heterogeneity, as well as the presence of cavities, holes, ruptures, voids, fractures, fissures, nooks, and crannies. These attributes play a critical role in enhancing the sorption capacity of the surface. Additionally, the high temperatures experienced during the carbonization and calcination processes result in the release of volatile matter from the microstructure of the particles. This phenomenon leads to the creation of voids on the surfaces of the adsorbent, ultimately transforming into pores. Generally, the heterogeneous surfaces of the adsorbent provide an advantageous setting for the adsorption of various pollutants from water and wastewater (Albatrni et al., 2022).

The full factorial design comprising four parameters at two levels was utilized in this research giving a total experimental runs of 32 in duplicate. The maximum removal efficiency attained in this study was reduced the initial dye concentration from 100 to 0.001 giving a maximum removal efficiency of 99.99%. This maximum removal efficiency was recorded at optimum treatment conditions of pH 9, contact time of 40 min, initial dye concentration of 100 mg/L and adsorbent dosage of 0.04 g/100 mL. On the other hand, the minimum removal efficiency recorded was determined to be 49.34%. The huge gap between the maximum and minimum removal efficacies indicates the significance of the effects of the operating parameters (Waghmare et al., 2023). Table 4 presents the adsorption capability of magnetic biochar derived from parthenium hysterophorus applied in the decontamination of MB from an aqueous solution. The maximum removal efficiency (99.99%) of MB attained utilizing magnetite doped biochar of parthenium hysterophorus is higher than the maximum removal efficiency (94%) attained using activated carbon derived from the same material (Fito et al., 2020). Magnetite doping enhances the adsorption capacity of Rumex abyssinicus derived biochar compared to plain biochar due to several factors. Firstly, the deposition of iron minerals on the carbon surface creates additional sorption sites, increasing the sorption ability of the biochar. Secondly, the presence of magnetite in the biochar composite leads to the activation of effective aggregates, resulting in improved adsorption performance for MB molecules (Xie et al., 2021; Guo et al., 2023).

The maximum adsorption capacity recorded in this study is compared with various magnetic biochars applied for the removal of MB from wastewater and aqueous solution. As a result, the recent study is found to be superior compared to many previous researches, providing a promising solution for the detoxification of MB saturated effluents. Table 5 presents a comprehensive comparative analysis of various biomass based magnetic biochars applied for the removal of MB.

In addition to individual effects on the adsorption performance of MB from onto magnetic biochar, it is crucial to consider the interaction effects among the operating parameters. In this regard, the batch adsorption performance data (Table 4) was fed to the design expert software version 11 and the interaction among different operating parameters was statistically analyzed. Accordingly, the batch adsorption performance data accurately fit to interactive regression model with adjusted and projected R² values of 0.9995 and 0.9983 respectively. Additionally, all the interaction between the operating factors is found significant (as their p-value is determined to be less than 0.05), which highlights the significant effect on the MB removal efficiency. Herein, the interaction of pH and initial MB concentration is found to have a negative effect on the removal efficiency of MB. Individually, the pH is determined to have an encouraging effect on the MB removal efficiency. Conversely, the individual effect of initial MB concentration art fixed values of contact time, adsorbent dosage and pH was determined to have a negative effect on the adsorption performance of MB dye. The interaction effect of pH and adsorbent dosage as well as pH and contact time is determined to positively affect the removal efficiency of the MB dye. On the other hand, the MB removal efficiency is negatively affected by the interaction of initial MB concentration and adsorbent dosage as well as adsorbent dosage and contact time. However, the interaction of initial MB concentration and contact time is found to have a positive impact on the MB removal efficiency. Table 6 presents the interactive model describing the adsorption pattern of MB adsorption onto magnetite biochar composite derived from parthenium hysterophorus. The relative impact of significant factors as their interaction on MB removal efficiency is presented in Eq. 6, whereas Eq. 7 presents the model equation for the removal of MB onto magnetic biochar. C > B > D > BC > CD > BD > AC > AD > A > AB (7)

Removal efficiency = 74.52 + 0.3056 A-8.14 B + 10.52 C +4.74 D-0.2956 AB+0.8669 AC+0.6181 AD -1.87 BC +0.8819 BD-1.39 CD (9).

The Langmuir, Freundlich, and Temkin isotherms were visually depicted in Figures 13, 14, 15, respectively. The determination coefficients (R²) for the Langmuir, Freundlich, and Temkin isotherms were found to be 0.94, 0.98, and 0.97, respectively. Additionally, residual sum of squares (RSS) used for error analysis of data fit to each isotherm models is provided. The Freundlich isotherm provided the best fit for the data, indicating a multilayer and heterogeneous adsorption process. Consequently, the surface of the adsorbent was determined to be heterogeneous, and the adsorption of MB occurred on the active sites of the multilayer surface. Moreover, the Langmuir isotherm exhibited a Q_max value of 327.87 mg/g and a K_L value of 0.0003 g/L, showcasing the effectiveness of the adsorption process. On the other hand, the Langmuir isothermal feasibility (R_L) and the Freundlich isotherm constant related to intensity (1/n) were determined to be 0.99 and 0.095, respectively, indicating a favorable adsorption process. The recorded value of 1/n in this study was 0.397, which is low, suggesting higher heterogeneity. In a favorable Freundlich isotherm (i.e., n > 1), active sites with the highest binding energies are utilized first for less heterogeneous surfaces, followed by weaker sites for more heterogeneous surfaces. The present study yielded a higher Freundlich adsorption capacity (K_F) of 254.68 (mg/g)/(mg/L) ⁿ. Furthermore, a higher K_F value indicates a lower free energy requirement for the adsorption process. The Temkin isotherm constants K_T and B_T were determined to be 16.693 L/g and 90.27 J/mol, respectively. Overall, the isotherm study showed that the adsorption of MB onto magnetic biochar is favourable with multilayer and heterogeneous surface interactions.

The kinetics of dye adsorption was investigated using the optimum conditions determined from the batch experiments. Table 7 presents the calculated adsorption capacities (Qe, cal), experimental adsorption capacities (Qe, exp), and values of parameters such as K₁, K₂ and K_P for all three kinetic models. The pseudo second order reaction adsorption kinetics exhibited a coefficient of determination (R²) of 0.99, while the pseudo first order reaction kinetics showed a lower value of R² (0.91). Therefore, given the higher coefficient of determination and the negligible difference between the equilibrium and calculated adsorption capacities (Qe, exp) and (Qe, calc), it can be concluded that the pseudo second order model provides a better description of the cationic dye adsorption kinetics. Moreover, the rate–limiting adsorption mechanism was found to be chemisorption, which occurs due to the formation of chemical bonds between the adsorbate molecules and specific surface locations (active sites) on the magnetic biochar. Additionally, an intraparticle diffusion model was developed to examine the adsorbate’s ability to penetrate the interior surface of the adsorbent. The intraparticle diffusion rate constant (K_P) and the C (mg/g) values were determined to be 1.25 mg/(g.min^0.5) and 64.27 mg/g respectively. These findings indicate that the adsorption rate is influenced by both the adsorbate and the adsorbent and that intraparticle diffusion does not solely control the adsorption process.

The adsorption thermodynamics of MB adsorption onto magnetic biochar derived from parthenium hysterophorus was investigated employing the Van’t Hof equation and the result of the analyses is presented in Figure 16. The thermodynamic parameters provide insights into the spontaneity, endothermicity, and feasibility of the adsorption process for methylene blue onto the magnetite-doped biochar composite. The values of ΔG indicate the feasibility of the process, with negative values indicating a spontaneous adsorption process. The values of ΔH indicate the endothermic nature of the process, with positive values indicating that heat is absorbed during adsorption. The values of ΔS indicate the increase in disorder during the adsorption process, with positive values indicating an increase in randomness. From the thermodynamics study, it can be observed that the value of ΔG was determined to have negative values throughout the study. Moreover, the values for both ΔH and ΔS were determined to be positive. Overall, the combination of negative ΔG, positive ΔH, and positive ΔS suggests that the adsorption process is spontaneous, endothermic, and feasible for the removal of MB onto the magnetite-doped biochar of parthenium hysterophorus.

The assessment of reusability over multiple cycles provides insight into the practical applicability and sustainability of magnetite biochar derived from Parthenium hysterophorus as an adsorbent for industrial-scale decolorization of methylene blue wastewater effluent. The ability of the magnetite biochar to maintain its adsorption capacity and efficiency over multiple cycles demonstrates its potential for practical use in wastewater treatment. The reusability of the magnetite biochar is attributed to its stability and the presence of magnetic nanoparticles, which allow for easy separation and regeneration of the adsorbent. The biochar-magnetite composite derived from parthenium hysterophorus was tested for MB removal for five times, evaluating the adsorption efficiency during adsorption-desorption cycles. In the initial stage, the adsorption efficiency was recorded as 99.8%, but decreased to 97.6% by the fifth cycle as shown in Figure 17. This decline indicates a slight reduction in adsorption performances as the number of cycles increased. The loss of active sites associated with incomplete desorption likely contributed to this decrease in performance. The reusability performance of magnetic biochar derived from parthenium hysterophorus is further presented in adsorption capacity as shown in Figure 18. As can be observed from Figure 18, the adsorption capacity for five consecutive cycles of reusability ranged from 249.5 to 244 mg/g. Additionally, the insignificant decrease in removal capacity after each cycle, demonstrates the adsorbent’s ability to be regenerated and reused. These findings confirm that the biochar-magnetite composite is highly reusable and stable, making it a promising candidate for practical applications in MB removal.